The present invention relates to brazing generator armature winding bars to hydraulic header clips, and to a method for sealing an armature winding bar to its header clips to prevent or reduce corrosion due to coolant water flowing through the end fitting and the armature winding bar.
The armature windings on large steam-turbine generators are generally water-cooled. The armature windings comprise an arrangement of half coils or armature bars (collectively referred to as “armature bars” or “bars”) connected at each end through copper or stainless steel fittings and water-cooled connections to form continuous hydraulic winding circuits.
Water-cooled armature winding bars are comprised of a plurality of small rectangular solid and hollow copper strands arranged to form a bar. The rectangular copper strands are generally arranged in rectangular bundles. The hollow strands each have an internal duct for conducting coolant through the bar. The ends of the strands are each brazed to a respective hydraulic header clip. The hydraulic header clip serves as both an electrical and a cooling flow connection for the armature winding bar.
The hydraulic header clip is a hollow connector that includes an enclosed chamber for ingress or egress of a cooling liquid, typically deionized water. At one open end, the clip encloses the ends of the copper strands of the armature winding bar. A braze alloy bonds the end sections of the strands to each other and to the hydraulic header clip. The braze joints between adjacent strand ends and between the strand ends and the clip should retain hydraulic and electrical integrity for the expected lifetime of the winding. A typical life time of a winding is on the order of tens of years.
Internal surfaces of the brazed joints between the clip and the ends of the strands are constantly exposed to the deionized, oxygenated water flowing through the clip and the hollow strands. The exposure of the brazed surfaces to the coolant can result in corrosion of the armature winding bar and hydraulic header clip. Corrosion tends to occur in the crevices of the joints between the hydraulic header clip and the strand ends of the armature bar, and in the crevices between the strand ends. Corrosion of a phosphorus-containing braze alloy and adjoining copper strand surfaces can occur if critical crevice geometry and crevice water chemistry conditions are present. Certain conditions promote crevice corrosion in the braze joints, such as: phosphorus, copper, suitable corrosion initiation sites and water. If any one of these conditions is eliminated from the clip to bar joints, crevice corrosion should be reduced or eliminated.
The corrosion process can initiate if the braze joint surfaces contain surface crevices, pinholes, or porosity at or near the surface of the joint and the critical water chemistry conditions that support corrosion. The corrosion process can progress through the braze joints especially when critical crevice geometry and water chemistry conditions exist. Porosity within the braze joints can accelerate corrosion. If allowed to progress through a joint, corrosion will eventually result in a water leak through the entire effective braze joint length and compromise the hydraulic integrity of the clip-to-strand joint. Accordingly, there is a long felt need for a corrosion-resistant clip-to-strand braze joint. The benefits of crevice corrosion-resistant braze joint are expected to include improved generator availability and generator reliability.